1. Field of the Invention
The present invention relates to an automotive air conditioning system. More particularly, it relates to a control apparatus for controlling operation of an automotive air conditioning system which includes an externally controlled, variable capacity-type refrigerant compressor.
2. Description of the Related Art
Control apparatus for controlling operation of an automotive air conditioning system, which include an externally controlled, variable capacity-type refrigerant compressor, are well known in the art. In prior art embodiment. The capacity of a refrigerant compressor is adjusted to control air temperature T.sub.e immediately downstream from an evaporator during operation of an automotive air conditioning system. T.sub.e is maintained at the set temperature T.sub.set by sending an electric signal having an amperage, which is determined by the proportional-plus-integral-plus-derivative control action (hereinafter "PID control action") of a conventional control apparatus, to an externally controlled, variable capacity control mechanism of the compressor.
In general, the operation of the automotive air conditioning system is divided into a first situation in which the automotive air conditioning system is operated in a static thermodynamic condition of the evaporator and a second situation in which the automotive air conditioning system is operated in a dynamic thermodynamic condition of the evaporator. In the first situation, heat load on the evaporator is slightly increased or decreased in response, for example, to the slight changes in the rotational speed of an evaporator fan caused by slight changes in electric load on the automobile's battery, or by slight changes in air temperature immediately upstream from the evaporator. On the other hand, in the second situation, heat load on evaporator is quickly increased or decreased by large amounts in response, for example, to changes in the rotational speed of the evaporator fan, e.g., changes in the amount of air flow which passes through an exterior surface of the evaporator, or a change in the automotive air conditioning mode, such as a switch from a passenger compartment air circulation mode to an outside air intake mode or vice versa.
In the conventional automotive air conditioning system described above, a coefficient of the PID control action of the control apparatus is fixed at one constant value during operation of the automotive air conditioning system. If, however, the coefficient of the PID control action of the control apparatus is selected at one constant value to effectively control the first situation of the operation of the automotive air conditioning system, the air temperature T.sub.e overshoots the set temperature T.sub.set by a significant amount. This amount is significant enough to lengthen the time period required for the air temperature T.sub.e to approach the set temperature T.sub.set in the second situation of the operation of the automotive air conditioning system. Such a delay is indicated by a dashed line in FIG. 4. On the other hand, if the coefficient of the PID control action of the control apparatus is selected at another constant value to effectively control the second situation of the operation of the automotive air conditioning system, the air temperature T.sub.e can not be maintained at the set temperature T.sub.set, in the first situation of the operation of the automotive air conditioning system. This is due to the oversensitive control of the operation of the automotive air conditioning system.